Inorganic ions are an essential requirement for life and are found in large amounts in drinking water, blood and every cell of an organism as well as in the environment. For example, the concentration of many ions i.e. sodium, potassium, magnesium, and calcium inside and outside of cells is essential for any living organism. Consequently, the ion concentration in the blood and blood cells of animals and human beings also is of high importance for a large variety of body functions.
Normally lithium is a trace element present in the blood plasma, but it is used as a drug to treat bipolar mood disorder. It is estimated that worldwide over one million people take lithium on a daily basis. A disadvantage in the use of lithium is the very low therapeutic index, i.e., the ratio between the toxic concentration and the therapeutic concentration. Most patients respond well to a blood plasma concentration of 0.4-1.2 mmol/L lithium while toxic effects can occur at a lithium concentration of above 1.6 mmol/L. A prolonged high blood lithium level can even result in permanent damage to the nervous system and even death. Monitoring of the lithium concentration during treatment is therefore essential, with regular checks every couple of months to keep the lithium level at desired level.
To avoid extensive operator handling, ion-selective electrodes (ISEs) are routinely used for measurements of blood parameters in an automated fashion. These ISEs are fast and offer a large dynamic range; however, their response is logarithmic and the required high selectivity for lithium can be a problem. Additionally, in case of lithium intoxication a fast procedure for blood analysis is required. Currently, a venous blood sample must be withdrawn from the patient by specially trained personnel and transported to the central laboratory and the blood cells need to be removed before the measurement is made. This procedure can take up to 45 minutes. To minimize sample throughput time and enable measurements on location, miniaturized devices employing ion-sensitive field-effect transistors are available to determine the concentration of potassium and sodium in whole blood even as a hand-held analyzer. However, such analyzers are not used for lithium determination, because of the high background concentration of other charged species, in particular sodium ions, compared to the much smaller concentration of lithium ions.
The direct measurement of lithium in whole blood and the determination of inorganic cations in blood plasma have been described and demonstrated by E. Vrouwe et al. in Electrophoresis 2004, 25, 1660-1667 and in Electrophoresis 2005, 26, 3032-3042. Using microchip capillary electrophoresis (CE) with defined sample loading and applying the principles of column coupling, alkali metals were determined in a drop of whole blood. Blood collected from a finger stick was transferred onto the chip without extraction or removal of components. The lithium concentration can be determined in the blood plasma from a patient on lithium therapy without sample pre-treatment. Using a microchip with conductivity detection, a detection limit of 0.1 mmol/L has been obtained for lithium in a 140 mmol/L sodium matrix.
In these disclosures, the components of the blood sample are separated electrophoretically inside a micro-channel. A double T injection geometry is used to select the ion components of interest and to guide them to detection electrodes.
In these systems, the sampling loading has to be well defined in order to ensure the correct separation of blood plasma components in the double T geometry. In addition, the double T geometry is complicated to apply and not well suited for easy to use applications.